Searching for and digging out oil, coal, or natural gas are not the only ways that fossil fuels harm the environment.The combustion of these fuels has impacts as well, largely through air pollution.

Today's air pollution comes primarily from two sources: automobiles in cities and the combustion of coal in electric power plants.The smog caused by automobiles is the most notorious pollution because it spreads through cities as a brown haze, visible to everyone. In contrast,many power plants are located some distance away from urban centers so that most people see them only rarely. Both sources, however, contribute pollutants.

Smog is found in large cities such as Los Angeles, Denver, and Houston. It has its origins in automobile exhaust, which produces particles and volatile gases that react with one another in sunlight (often...